Apparatus and method for treating used electroless plating solutions

ABSTRACT

A method and apparatus for treating an electroless plating solution is provided. The apparatus includes a reaction vessel, a cathode and an anode disposed in the interior of the reaction vessel and in electrical communication with a power source, a drain disposed in the reaction vessel, and a nozzle in fluid communication with the drain, disposed in the reaction vessel such that the nozzle and the drain are separated by the cathode and the anode. The method includes disposing an electroless plating solution in the reaction vessel, recirculating the plating solution through the reaction vessel by draining the plating solution from the reaction vessel and subsequently re-injecting the plating solution into the reaction vessel through the nozzle. A current is driven through the anode and cathode to oxidize reducing agents in the liquid and plate out the metal as the elemental metal.

FIELD OF THE INVENTION

The present invention relates to a process and apparatus for recoveringand treating used electroless plating solutions by simultaneouselectrolysis and electrowinning.

BACKGROUND

Electroless plating is a commonly used method for introducing a metalcoating onto an object. To coat an object with a metal by electrolessplating, a metal compound is placed in solution and the elemental metalis subsequently deposited via a chemical reaction. Electroless platingmay be used to provide a highly uniform coating of a metal such asnickel, copper, silver, gold, platinum, or palladium on an item.Electroless plating is frequently used in the electronics industry, forexample, in the processing of semiconductor wafers.

With time and use, an electroless plating solution will become exhaustedand/or contaminated with by-products of the plating process,necessitating its replacement. Spent plating solutions, however, containmetal compounds, with their environmental considerations. Spent platingsolutions can also tend to evolve a significant amount of hydrogen gas,presenting an explosion and fire hazard. As a result, a variety ofmethods have been devised to treat spent plating solutions.

U.S. Pat. No. 6,391,209, the disclosure of which is incorporated hereinby reference, describes a number of prior methods for the treatment ofspent plating solutions. These include treatment of the solution with anoxidizing agent such as hydrogen peroxide. Another method includes thechemical reduction of the metal and subsequent precipitation of organiccomplexing agents. Plating solutions may also be treated by exposure toozone, ultraviolet light, or hydrogen peroxide, or a combinationthereof.

U.S. Pat. No. 5,730,856, the disclosure of which is also incorporatedherein by reference, describes a method for treating electroless platingsolutions by electrolytic oxidation and simultaneous vibration andfluidization by an oscillating stirrer.

Electrochemical cells have also been used to remove metals from metalcontaining solutions such as electroless plating solutions. U.S. Pat.No. 6,162,333 to Lemon et al., the disclosure of which is incorporatedherein by reference, describes such a cell.

SUMMARY

A method for treating an electroless plating solution is provided, themethod comprising:

-   -   providing a reaction vessel containing an anode, a cathode, a        drain and a nozzle, wherein the nozzle is in fluid communication        with the drain;    -   disposing the electroless plating solution in the reaction        vessel such that the anode and the cathode are at least        partially immersed in the plating solution;    -   recirculating the plating solution through the reaction vessel        by draining the plating solution from the reaction vessel        through the drain and subsequently re-injecting the plating        solution into the reaction vessel through the nozzle;    -   placing the anode and cathode in electrical communication with a        power source and driving an electrical current through the anode        and the cathode to produce a treated plating liquid.

The method may additionally comprise sparging the reaction vessel withan inert gas to create a sparge gas. The method may further compriseremoving residual liquid from the sparge gas and venting the sparge gasto a hydrogen gas scrubber. The inert gas may consist essentially ofnitrogen gas. Typically, the anode may comprise steel and the cathodemay comprise brass.

An apparatus for treating an electroless plating liquid is provided, theapparatus comprising:

-   -   a reaction vessel;    -   a cathode and an anode in electrical communication with a power        source, wherein the cathode and anode are disposed in the        interior of the reaction vessel;    -   a drain disposed in the reaction vessel;    -   a nozzle in fluid communication with the drain, disposed in the        reaction vessel such that the nozzle and the drain are separated        by the cathode and the anode.

Additionally, the apparatus may additionally comprise a gas sparger incommunication with an inert gas source. In such an embodiment, thereaction vessel is typically vented and is in communication with ahydrogen gas scrubber. In one example, inert gas consists essentially ofnitrogen gas. The reaction vessel may additionally comprise a heatexchanger.

The apparatus may typically contain a steel anode and a brass cathode.

Additionally, the apparatus may comprise a metal compound-restrictivefilter disposed in the reaction vessel such that only liquid essentiallyfree of metal passes through the filter and into the drain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an apparatus for treating an electrolessplating liquid

DETAILED DESCRIPTION

One embodiment of an apparatus for treating an electroless platingliquid may be described with reference to FIG. 1. A treatment apparatus10 comprises a reaction vessel 12. In the interior of reaction vessel 12is a cathode 14 and an anode 16. Cathode 14 may be brass and anode 16 isany metal that is not oxidized in the process, such as stainless steel.The anode and cathode are in electrical communication with a powersource, in certain embodiments, a direct current power source. In oneparticular embodiment, the current applied is between about 1 and about10 amperes, although other current levels may also be used.

Also contained within reaction vessel 12 is a drain 18 and a nozzle 20,such as a spray nozzle. Drain 18 is in fluid communication with a firstvalve 19, which is in turn, in fluid communication with a pump 22. Pump22 is in fluid communication with nozzle 20 via a second valve 21.Within reaction vessel 12, nozzle 20 and drain 18 are separated by atleast the distance between the anode 16 and cathode 14. Reaction vessel12 may additionally include a sparger in fluid communication with asource of inert gas 30. The inert gas may be, for example, nitrogen, ora noble gas, such as helium or argon. Reaction vessel 12 may alsoinclude a vapor trap 24 and a heat exchanger 26 such as a cooling jacketor coiled tube through which cooled water is circulated. If a sparger ispresent, then a vent 32 is also present. In certain embodiments, vent 32is preferably in fluid communication with a hydrogen gas scrubber (notshown).

Reaction vessel 12 may optionally comprise a metal compound-restrictivefilter immediately upstream of drain 18, thereby concentrating metalcompounds in the reaction vessel and preventing them from leaving thereaction vessel. Such a filter can increase the overall efficiency ofthe treatment process, described in more detail below.

The operation of the apparatus may also be described with reference toFIG. 1. An electroless plating solution is disposed in reaction vessel12 to a predetermined level. Typically, the anode and cathode will be atleast partially immersed in the plating solution to be treated. Firstvalve 19 may be in fluid communication with a plating solution source(not shown) and second valve 21 may be set to distribute liquid frompump 22 to nozzle 20. When a predetermined amount of plating liquid isdisposed in reaction vessel 12, valve 19 may be switched to receiveliquid from drain 18. Liquid is drained from drain 18, and pumped bypump 22 to nozzle 20, where the liquid is re-injected into reactionvessel 12, thereby providing circulation of the plating liquid duringtreatment.

During treatment of the plating liquid, the liquid is typically spargedwith an inert gas, such as nitrogen, helium or argon. Hydrogen gas thatis released during the treatment of the plating liquid, is then sweptaway with the inert gas to create a sparge gas. Residual amounts ofliquid, introduced for example, by liquid injected into the reactionvessel by nozzle 20, and carried with the sparge gas, is removed byvapor trap 24. The at least partially dried sparge gas is then ventedthrough vent 32, in certain embodiments to a hydrogen gas scrubber (notshown).

An electric current is driven through the anode and the cathode. Thecurrent may be a direct current. The anode may be steel, such asstainless steel, and the cathode may be brass. Reducing agents in theplating solution are oxidized at the anode in an electrolysis reaction,decreasing the volume of hydrogen gas generated during treatment,although some hydrogen gas may still be generated from hydrolysis ofwater or the oxidation of chemical constituents of the plating solution.The metal salt or salts in the plating solution are reduced at thecathode in an electrowinning reaction, causing the metal to plate ontothe cathode as the elemental metal. Heat may also be generated duringtreatment which may be removed by heat exchanger 26. The temperature ofthe plating liquid may be maintained at a temperature suitable fordischarge or further treatment. For example, the plating solution may bemaintained at about 50° C. or less during treatment. The plating liquidmay be further treated, for example, by contacting it with an ionexchange resin after discharge from the system.

The treatment continues for a sufficient time and under sufficientcurrent to oxidize substantially all of the reducing agents. Progress ofthe reaction may be monitored by oxidation-reduction potential (ORP),colorimetry (if one of the species absorbs visible or ultravioletlight), or other known methods. The treatment may continue undersufficient conditions to oxidize all of the reducing agents according tostoichiometric calculations. For example, if the concentration of areducing agent is 1 g/liter, and the reducing agent has a molar mass of58 g/mol and gives up 6 electrons per molecule in the reduction process,complete electrolysis would take 33 minutes at an applied current of 5amps (assuming 100% current efficiency):1 g÷(58 g/mole)×(6 mole e⁻/mole)×(96,500 coulombs/mole e⁻)÷(5coulomb/sec)÷(60 sec/min)=33 min.

When treatment is complete, valve 21 is switched to evacuate the treatedplating liquid from reaction vessel 12.

EXAMPLE

The following example is set forth to further illustrate an embodimentof the process. The example should not be construed as limiting theprocess in any manner.

A used electroless plating solution containing cobalt ions as theoxidizing agent and dimethylamineborane (DMAB) as the reducing agent maybe treated to prevent plate out and spontaneous production of hydrogengas as follows.

A used electroless plating solution is placed in reaction vessel 12 andrecirculated as described above. Upon application of electric current,the oxidation reaction at the anode is: DMAB→DMA+B(OH)₃+6e⁻; thereduction reaction at the cathode is: Co⁺²+2e⁻→Co. The used electrolessplating solution is treated until at least a majority of the reducingagent is oxidized and at least a majority of the metal ions are reduced,preferably to an elemental state. Preferably, the reactions arecontinued until substantially all the of the reducing agent is oxidizedand the metal ions are reduced. The used electroless plating solution isthen removed from the apparatus.

The result is a used electroless plating solution from which the cobaltions have been removed and in which the DMAB has been oxidized, so thatthe solution will not plate out and will not spontaneously producehydrogen gas.

The present apparatus and process are advantageous over prior systemsbecause they provide for the removal of metals and metal compounds froma plating liquid rapidly and economically with a minimum generation ofhydrogen gas.

The entire process may be controlled by a programmable controller, andrecords data from the process can be sent to a computer which can beused to retrieve the data remotely. The apparatus and process mayinclude a fully automated microprocessor controller which continuouslymonitors system operation providing fault detection, pressure and/ortemperature control and valve sequencing, ensuring reliability, whileminimizing operator involvement.

The apparatus may include system alarms to detect potential hazards,such as temperature or pressure excursions, to ensure system integrity.Alarm and warning conditions may be indicated at the operator interfaceand may be accompanied by an alarm beeper.

It will be understood that embodiments(s) described herein is/are merelyexemplary, and that one skilled in the art may make variations andmodifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as described hereinabove. Itshould be understood that any embodiments described hereinabove are onlyin the alternative, but can be combined.

1. A method for treating an electroless plating solution, the methodcomprising: providing a reaction vessel containing an anode, a cathode,a drain and a nozzle, wherein the nozzle is in fluid communication withthe drain; disposing the electroless plating solution in the reactionvessel such that the anode and the cathode are at least partiallyimmersed in the plating solution; recirculating the plating solutionthrough the reaction vessel by draining the plating solution from thereaction vessel through the drain and subsequently re-injecting theplating solution into the reaction vessel through the nozzle; placingthe anode and cathode in electrical communication with a power sourceand driving an electrical current through the anode and the cathode toproduce a treated plating liquid.
 2. The method of claim 1, includingoxidizing plating solution reducing agents at the anode and reducingplating solution metal salt at the cathode.
 3. The method of claim 1,additionally comprising sparging the reaction vessel with an inert gasto create a sparge gas.
 4. The method of claim 3, wherein said spargingthe reaction vessel includes removing residual liquid from the spargegas and venting the sparge gas to a hydrogen gas scrubber.
 5. The methodof claim 3, wherein the inert gas consists essentially of nitrogen gas.6. The method of claim 1, wherein the anode comprises steel and thecathode comprises brass.
 7. The method of claim 1, wherein the platingsolution is maintained at a temperature of about 50 degrees C. or less.8. The method of claim 1, wherein the electrical current is betweenabout 1 and about 10 amperes.
 9. The method of claim 1, additionallycomprising exposing the treated plating liquid to an ion exchange resin.10. The method of claim 1, wherein the reaction vessel additionallycomprises a metal compound-restrictive filter disposed such that onlysubstantially metal particle-free liquid passes through the drain. 11.The method of claim 1, including monitoring the progress of thetreating.
 12. An apparatus for treating an electroless plating liquid,the apparatus comprising: a reaction vessel; a cathode and an anode inelectrical communication with a power source, wherein the cathode andanode are disposed in the interior of the reaction vessel; a draindisposed in the reaction vessel; a nozzle in fluid communication withthe drain, disposed in the reaction vessel such that the nozzle and thedrain are separated by the cathode and the anode.
 13. The apparatus ofclaim 12, additionally comprising a gas sparger in communication with aninert gas source.
 14. The apparatus of claim 13, wherein the reactionvessel is vented and in communication with a hydrogen gas scrubber. 15.The apparatus of claim 12, wherein the reaction vessel additionallycomprises a heat exchanger.
 16. The apparatus of claim 12, wherein theanode comprises steel and the cathode comprises brass.
 17. The apparatusof claim 12, additionally comprising a metal compound-restrictive filterdisposed such that only liquid substantially free of metal particlespasses through the filter and through the drain.